As semiconductor integrated circuit device feature sizes shrink, high-performance and reliable interconnect technology using copper-based metallization becomes increasingly important. However, copper interconnect technology faces a number of important process integration and manufacturing challenges. For instance, copper diffuses relatively rapidly through many materials, including both metals and dielectrics, particularly, at temperatures above .about.300.degree. C. In a typical device, copper diffusion into the inter-metal dielectric (IMD) such as silicon dioxide results in current leakage between adjacent metal lines (i.e., line-to-line leakage) and degradation of Inter-level dielectric (ILD) breakdown field. If copper diffuses through the IMD and the pre-metal dielectric (PMD) into the device transistor region, device performance and reliability degrade significantly and the device may become nonfunctional. In addition, copper is prone to corrosion and generally must be passivated to maintain its electrical conductivity characteristics.
Difficulties with the forming of copper interconnects have lead to the development of barrier materials that separate the copper metallization regions from vulnerable device regions. These barrier materials hinder the diffusion of copper into the vulnerable regions. Effective barrier materials generally must possess several characteristics. One important characteristic is a low diffusion coefficient for copper. Copper tends to diffuse during thermal cycling, such as thermal cycling experienced by a substrate during multilevel metallization processes, as well as during actual device operation under applied electric fields. Thus, barrier materials must generally remain thermally stable, including good structural stability so that the barrier remains effective during processing. Another important characteristic for a barrier material is that it generally must provide good adhesive interfaces for supporting deposition of copper on the barrier. Thus, the barrier material should have excellent adhesion to its underlying layer, such as oxide or low-k dielectric underlying layers, and provide excellent adhesion to a copper layer deposited on the barrier material. Further, the barrier should provide a good nucleation surface to promote &lt;111&gt; texture in the overlying copper layer such as overlying layers deposited by CVD, PVD, and/or electrochemical deposition (ECD).
Another important barrier characteristic is low electrical resistivity and via contact interface resistance. For instance, if a barrier is deposited between an underlying copper metal line and an overlying copper via plug, then the barrier should provide minimal increase of resistance for transmission of electric current between the underlying metal line and the via plug. As an example, amorphous barriers of refractory metals typically exhibit good diffusion barrier properties but also typically increase resistance between overlying and underlying copper layers in excess of acceptable levels. This increase is likely due to the relatively high resistivity of the barrier layer.
Another important characteristic of a barrier material is that its deposition should occur with good step coverage in high-aspect-ratio device features such as the dual-damascene trench and via structures. Barrier thickness on feature sidewall and bottom surfaces should be comparable to barrier thickness in the field, and barrier structure should be invariant with wafer topography. As an example of the importance of good step coverage, consider deposition of a barrier with a minimum thickness of 75 .ANG. needed to prevent copper diffusion. If deposition of the barrier is accomplished with 25% step coverage, then a barrier thickness of 300 .ANG. is needed to insure that a minimum thickness of 75 .ANG. is accomplished throughout the interconnect structure. By comparison, if step coverage of the barrier is 75%, then a minimum barrier thickness of 75 .ANG. can be accomplished with barrier deposition of a thickness of 100 .ANG. over the field region.
In an attempt to meet the requirements for copper metallization barriers, a number of advanced barrier materials have been developed to supplant traditional barriers used for aluminum and tungsten metallization, such as TiN and TiW. For instance, Ta, TaN, NbN, WN.sub.x and ternary barriers such as TiSiN, TaSiN, WSiN, and WBN all support copper metallization with varying degrees of success. However, these materials are generally deposited with physical vapor deposition (PVD) which provides limited step and bottom coverage, putting the usefulness of these (PVD) barrier materials in doubt as device dimensions continue to shrink.